1. Field of the Invention
The present invention relates to an optical transmission system that adopts a variable optical attenuation (VOA) function. More specifically, the present invention relates to an optical transmission system that uses a micro mirror such as a micro electro mechanical systems (MEMS) mirror for reflecting an optical signal to enter an optical fiber and controls an angle of the micro mirror so that quantity of incident light is altered for controlling attenuation of the optical signal.
2. Description of the Prior Art
Conventionally, a micro mirror utilizing the MEMS technology, i.e., a digital micro-mirror device (DMD) is developed and is used for an optical switching system that is installed in a node of an optical network (see Japanese unexamined patent publication No. 2005-99682). The optical switching system includes a plurality of micro mirrors having reflection planes whose angles can be controlled and that are arranged on a plane. Optical signals that enter a plurality of input ports are reflected by the plurality of micro mirrors and are led to selected corresponding output ports among a plurality of output ports. Since the ports are switched in this way, it is able to perform optical exchange of a plurality of channels of optical signals on an optical transmission path.
Furthermore, the optical transmission system is provided with an optical level attenuation function for adjusting intensity of the optical signal to be transmitted. FIG. 10 is a perspective view showing an example of a structure of a conventional optical transmission system 80 equipped with the optical level attenuation function, FIG. 11 is a front elevation of the optical transmission system 80 shown in FIG. 10, FIG. 12 is a diagram showing a principle of the optical level attenuation function, and FIG. 13 is a diagram showing a tolerance curve in the conventional optical transmission system 80.
In FIGS. 10 and 11, the optical transmission system 80 includes an MEMS mirror 81, a collimator lens 82, and an optical fiber 83. The optical signal HS that is incident light toward the mirror 81 is reflected by the mirror 81 and enters the collimator lens 82. Then, it propagates in the optical fiber 83 and is transmitted to the outside. In this case, if the angle θ of the mirror 81 is adjusted precisely, the incident position of the optical signal HS to the collimator lens 82 is altered.
In FIG. 12, if the center of the optical signal HS enters the center position PS1 of the collimator lens 82, attenuation (optical attenuation) becomes minimum so that the optical signal HS having the largest intensity is transmitted to the optical fiber 83. However, as the center of the optical signal HS is shifted to the edge portion of the collimator lens 82, attenuation increases so that intensity of the optical signal HS that is transmitted to the optical fiber 83 is decreased. For example, the attenuation becomes very large in the position PS2 where the center of the optical signal HS is out of the edge portion of the collimator lens 82.
As a result, a relationship between a control angle θ applied to the mirror 81 and the attenuation is a curve like an inverted parabola shown in FIG. 13. More specifically, if the center of the optical signal HS is in the vicinity of the center position PS1 of the collimator lens 82, a variation of attenuation (ΔDL8) with respect to a variation of control angle θ (Δθ8) is small. In contrast, as the center of the optical signal HS goes to the edge portion of the collimator lens 82, a variation of attenuation (ΔDL9) with respect to a variation of control angle θ (Δθ8) becomes large. In other words, as the optical signal HS goes to the edge portion of the collimator lens 82, the attenuation (ΔDL) increases together with an increase of the attenuation with respect to the same variation of the control angle θ so as to alter in an order of square approximately. In this way, in the conventional structure, attenuation (db) is substantially proportional approximately to square of the control angle θ.
As described above, the conventional optical transmission system 80 adjusts the optical path of the optical signal HS by the mirror 81, so that the attenuation is adjusted by the quantity of incident light to the collimator lens 82. Therefore, as an optical attenuation effect, quantity of incident light is attenuated in accordance with Gauss theorem with respect to characteristics of angle variation to control voltage applied to the mirror 81 and an angle. Therefore, the relationship between the control angle θ of the mirror 81 and the attenuation is not linear.
In the conventional structure, an apparent characteristic correction is performed by the control voltage to be applied to the mirror 81, which has a characteristic opposite to the curve shown in FIG. 13, e.g., a root characteristic, for example. In this case too, however, the relationship between variation of the control angle θ by the control voltage to be actually applied to the mirror 81 and variation of the attenuation is not linear.
Therefore, there is a problem that if the optical signal HS is positioned at the vicinity of the end portion of the collimator lens 82 for obtaining large attenuation, it is affected easily by a variation of the control voltage, a power source noise or an external noise, resulting in that a variation of intensity of the optical signal HS is generated easily.